len(all_sys) should > 0 for post_fp_cp2k (#1245)

fix for this discussion

#1237 (reply in thread)

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Han Wang <92130845+wanghan-iapcm@users.noreply.github.com>

dpgen/generator/run.py

@@ -4295,7 +4295,7 @@ def post_fp_cp2k(iter_index, jdata, rfailed=None):
                 all_sys.append(_sys)
 
         icount += len(all_sys)
-        if all_sys is not None:
+        if (all_sys is not None) and (len(all_sys) > 0):
             sys_data_path = os.path.join(work_path, "data.%s" % ss)
             all_sys.to_deepmd_raw(sys_data_path)
             all_sys.to_deepmd_npy(sys_data_path, set_size=len(sys_output))